1. Field of the Invention
The present invention relates to a fuel property determination system, and a fuel property determination method for an internal combustion engine.
2. Description of the Related Art
As fuels for internal combustion engines, there are heavy fuels, which are difficult to vaporize, and light fuels, which are easy to vaporize. When the engine is idling after start up, the fuel injection amount is controlled so that the idling speed is maintained at a desired value. After the warm-up of the engine, since even a heavy fuel can be sufficiently vaporized, air-fuel ratio control is executed based on the output from an air-fuel ratio sensor disposed in the engine exhaust system. Therefore, after the warm-up of the engine, fuel is not excessively supplied to the internal combustion engine, whether a heavy fuel or a light fuel.
When a heavy fuel is unintentionally supplied to an internal combustion engine during the engine starting process from cranking until the engine speed reaches the idling speed, the fuel cannot be sufficiently vaporized, which makes it difficult to start the engine. Therefore, in general, the fuel injection amounts during the engine starting process are set on the assumption that a heavy fuel will be injected.
When a light fuel is injected in spite of the fuel injection amounts set as above, an excessive amount of fuel is injected, and a relatively large amount of unburned fuel is discharged into the engine exhaust system. Therefore, it is desired to correct the fuel injection amounts during the engine starting process based on the fuel property. An art for determining the property of fuel to be injected from a fuel injection valve after the start of engine has been proposed (see JP-A-Hei 9-256898, for example).
However, even when the fuel property is determined after the start of engine, if refueling is thereafter done before the stop of the engine, the determined fuel property is changed from the actual fuel property. When fuel injection amounts are corrected based on a fuel property different from the actual fuel property at the next start of the engine, the engine may be difficult to start, or a large amount of unburned fuel may be discharged during the engine starting process.